1. Field of the Invention
The present invention relates to a reflective-type liquid crystal display (LCD) and a method for manufacturing a same and more particularly to the reflective-type LCD having a layer-stacked type xc2xc wavelength plate formed by combining a xc2xd wavelength phase difference film with a xc2xc wavelength phase difference film placed on one side opposite to another side being in contact with a liquid crystal in a facing substrate on the obverse side of a display and opposing to a thin film transistor (TFT) substrate, and to the method for manufacturing the above reflective-type LCD.
The present application claims priority of Japanese Patent Application No.2001-022485 filed on Jan. 30, 2001, which is hereby incorporated by reference.
2. Description of the Related Art
An LCD is widely used as a display unit for a variety of information devices or a like. Such the LCD is basically configured so that a liquid crystal is sandwiched between a thin film transistor (TFT) substrate on which a TFT operating as a switching element adapted to drive the liquid crystal and which serves as a liquid crystal driving element forming substrate is formed and a facing substrate placed so as to be opposed to the TFT substrate. The LCD is broadly classified into two types, one type being a transmission-type LCD in which a desired display is achieved by observing, from a side of the facing substrate, light incident on the liquid crystal from a side of the TFT substrate and another being a reflective-type LCD in which a desired display is achieved by having light incident on the liquid crystal from the side of the facing substrate reflect on the side of the TFT substrate and by causing the light to be emitted from the side of the facing substrate.
When the transmission-type LCD is compared with the reflective-type LCD, the former is inferior, in a point of reduction in power consumption, to the latter, since the former requires a light source such as a backlight to have light enter the liquid crystal from the side of the TFT substrate and also since a ratio of the power consumption of the entire LCD to that of the light source is as large as several ten percent. Therefore, in applications in which reduction in the power consumption is required, in particular, the reflective-type LCD is mainly employed.
However, the reflective-type LCD configured without use of a polarizer on the side of the facing substrate has a shortcoming in that, an actual value of retardation of a phase difference film placed on the side of the liquid crystal or on the side of the facing substrate changes due to a shift in a viewing angle when a display is observed from the side of the facing substrate and, as a result, a yellowish glare develops on the display, causing discomfort when viewing to users.
In an attempt to solve the above problem, a reflective-type LCD is disclosed, for example, in Japanese Patent No.3095005. The disclosed reflective-type LCD includes, as shown in FIG. 7, a TFT substrate 101 on which a TFT (not shown) operating as a driving element to drive a liquid crystal is formed, a facing substrate 102, and a liquid crystal 103 sandwiched between the TFT substrate 101 and the facing substrate 102.
The TFT substrate 101 has a first transparent insulating substrate 104 made up of glass or a like, on which the TFT (not shown) is formed on the side of the liquid crystal 103, a reflective electrode 105 formed on the side of the liquid crystal 103 on the first transparent insulating substrate 104 and operating as a pixel electrode and also serving as a reflective plate and a first oriented film 106 formed in a manner so as to cover the reflective electrode 105 and to be in contact with the liquid crystal 103. The facing substrate 102 includes a second transparent insulating substrate 108 made up of glass or a like, a polarizer 109 formed on a side opposite to a side being in contact with the liquid crystal 103 on the second transparent insulating substrate 108, a layer-stacked type xc2xc wavelength plate 110 formed between the polarizer 109 and the second transparent insulating substrate 108 and constructed by combining a xc2xd wavelength phase difference film 111 with a xc2xc wavelength phase difference film 112, both being made from a polycarbonate polymer or a polysulfone polymer, a common electrode 113 on a side of the liquid crystal 103 on the second transparent insulating substrate 108, and a second oriented film 114 formed in a manner so as to cover the common electrode 113 and to be in contact with the liquid crystal 103. Moreover, as the liquid crystal 103, a Twisted Nematic (TN) type liquid crystal is used.
Here, a twisted direction of the liquid crystal 103 occurring when the liquid crystal 103 is traced from the side of the facing substrate 102 to the side of the TFT substrate 101 relative to an oriented direction on the side of the facing substrate 102 on the liquid crystal 103 is defined as being xe2x80x9cpositivexe2x80x9d, the polarizer 109 is formed so that an angle xe2x80x9cxcex1xe2x80x9d formed by its polarized light absorbing axis and by the oriented direction is set to be within a range of 5 degrees to 35 degrees, the xc2xd wavelength phase difference film 111 is placed so that an angle xe2x80x9cxcex2xe2x80x9d formed by its optical axis and by the oriented direction is set to be within a range of xe2x88x9215 degrees to 15 degrees and the xc2xc wavelength phase difference film 112 is placed so that an angle xe2x80x9cxcex3xe2x80x9d formed by its optical axis and by the oriented direction is set to be within a range of xe2x88x9275 degrees to xe2x88x9245 degrees (refer to FIG. 2). Moreover, a twisted angle of the liquid crystal 103 employed in the example is set to be within a range of 66 degrees to 74 degrees and a product xe2x80x9cxcex94 ndxe2x80x9d of a refractive index anisotropy (angle) of the liquid crystal 103 and a thickness of a layer of the liquid crystal 103 is set to be within a range of 0.21 xcexcm to 0.31 xcexcm.
In the conventional reflective-type LCD having configurations described above, since a change in retardation caused by a shift in a viewing angle of the liquid crystal 103 and a change in retardation caused by a shift in viewing angles of the phase difference films 111 and 112 can totally cancel each other out, development of unwanted colors on a display due to the change in the viewing angle can be resolved.
However, the conventional reflective-type LCD has another problem. That is, since the conventional reflective-type LCD uses, as the phase difference film to be formed on the side of the facing substrate 102, a material exhibiting great wavelength dispersion in anisotropy of the refractive index in a visible light region, other colors develop at the same time when a black color is displayed. In the conventional reflective-type LCD, a polycarbonate polymer or polysulfone polymer is used as the xc2xd wavelength phase difference film 111 and as the xc2xc wavelength phase difference film 112 both making up the layer stacked-type xc2xc wavelength plate 110 placed on a side opposite to a side being in contact with the liquid crystal 103 in the facing substrate 102, however, since these materials exhibit great wavelength dispersion in anisotropy of the refractive index, other colors develop at the same time when a black color is displayed.
FIG. 3 is a diagram explaining the wavelength dispersion in anisotropy of the refractive index occurring when the xc2xd wavelength phase difference film 111 and xc2xc wavelength phase difference film 112 both being made from the polycarbonate polymer are used in the conventional reflective-type LCD, in which a ratio of the wavelength dispersion is plotted as ordinate and a wavelength of light in a visible range as abscissa. The degree of the wavelength dispersion is indicated by a ratio of the refractive index anisotropy xe2x80x9cxcex94 n (xcex)xe2x80x9d in an arbitrary wavelength xe2x80x9cxcexxe2x80x9d to a refractive index anisotropy xe2x80x9cxcex94 n (550)xe2x80x9d in a reference wavelength (550 nm) in a green color light. Moreover, in FIG. 3, a comparison is made in the wavelength dispersion in anisotropy of the refractive index between the conventional reflective-type LCD and a reflective-type LCD of the present invention described later. A characteristic curve xe2x80x9caxe2x80x9d indicates a characteristic occurring when the polycarbonate polymer is used as a material for each of the phase difference films 111 and 112 (conventional example). A characteristic curve xe2x80x9cbxe2x80x9d indicates a characteristic occurring in the case of an embodiment of the present invention described later. Moreover, Tables 1 and 2 show comparisons in the ratio of the wavelength dispersion corresponding to an arbitrary wavelength xe2x80x9cxcexxe2x80x9d between the polycarbonate polymer and a norbornene polymer Arton(trademark) (trade name used by JSR Corporation, Japan, to be described later in the embodiment of the present invention). For example, in Table 1, when the polycarbonate polymer is used, the ratio of the wavelength dispersion corresponding to the wavelength of 500 nm is 1.016.
As is apparent from FIG. 3 and Tables 1 and 2, the characteristic curve xe2x80x9caxe2x80x9d shows that the shorter the wavelength, the larger the ratio of the wavelength dispersion and the greater the wavelength dispersion in anisotropy of the refractive index. This indicates that, since the retardation becomes large in a region of a blue color having a wavelength of about 430 nm, when a black color is to be displayed, a blue color light leaks and the blue color develops in the display. Therefore, when the black color is to be displayed, the black color cannot be faithfully displayed, causing unnatural display.
Moreover, FIG. 4 shows an XY chromaticity diagram in the conventional reflective-type LCD in which a region indicated by dashed lines is a chromaticity region xe2x80x9caxe2x80x9d surrounded by a red (R), green (G), and blue (B) colors. Points xe2x80x9cBLxe2x80x9d indicate coordinates for the black color. Here, a mark xe2x80x9c▪xe2x80x9d shows the characteristic occurring when the polycarbonate polymer is used as the material for each of the phase difference films 111 and 112 (in the conventional example) and a mark xe2x80x9cxe2x97xafxe2x80x9d shows the characteristic occurring in the case of the embodiment of the present invention. As is apparent from FIG. 4, in the conventional example, an area of the chromaticity region xe2x80x9caxe2x80x9d is relatively reduced. Moreover, in the conventional example, a value of the black color in an xe2x80x9cxxe2x80x9d coordinate is as relatively small as about 0.218 and a value of the black color in a xe2x80x9cyxe2x80x9d coordinate is also as relatively small as 0.240. For the reasons described above, in the conventional reflective-type LCD, a bluish black color develops inevitably in the display.
Moreover, another problem is that, in the conventional reflective-type LCD, since the twisted angle of the liquid crystal is set so as to be smaller than that of an ordinary TN liquid crystal, it is difficult to easily obtain high contrast. That is, in the conventional reflective-type LCD, as described above, the twisted angle of the liquid crystal is set to be within a range of 66 degrees to 74 degrees and the value is smaller than that of the ordinary TN liquid crystal being about 90 degrees, which makes it difficult for the liquid crystal to fully rise. In the widely-used normally-white type reflective-type LCD, it is ideal that the liquid crystal fully rises when the black color is displayed and the retardation is near to xe2x80x9c0xe2x80x9d (zero). Therefore, if the liquid crystal does not rise fully, since the retardation exhibited by the liquid crystal causes a phase of light passing through the liquid crystal to be changed, resulting in a rise in luminance of a black color, contrast becomes low.
To solve this problem, a method is available in which the driving voltage is increased to have sufficient contrast in the reflective-type LCD, however, this method causes the increase in power consumption and interferes with the reduction in power consumption.
In view of the above, it is an object of the present invention to provide a reflective-type LCD and a method for manufacturing the same which are capable of preventing development of an unwanted color on a display when a black color is displayed and of stably achieving high contrast.
According to a first aspect of the present invention, there is provided a reflective-type LCD for obtaining a desired display including:
a liquid crystal;
a liquid crystal driving element forming substrate;
a facing substrate;
wherein the liquid crystal is formed in a manner to be sandwiched between the liquid crystal driving element forming substrate and the facing substrate,
wherein light incident on the liquid crystal from a side of the facing substrate is reflected on a side of the liquid crystal driving element forming substrate and is emitted from a side of the facing substrate so as to be observed,
wherein the liquid crystal driving element forming substrate has an insulating substrate, a liquid crystal driving element formed on a side of the liquid crystal on the insulating substrate, a reflective electrode formed on the liquid crystal driving element, and a first oriented film formed in a manner to cover the reflective electrode,
wherein the facing substrate has a transparent insulating substrate, a common electrode constructed of a transparent conductor formed on a side of the liquid crystal on the transparent insulating substrate, a second oriented film formed in a manner so as to cover the common electrode, a polarizer formed on a side opposite to a side being in contact with the liquid crystal on the transparent insulating substrate, and a layer-stacked type xc2xc wavelength plate constructed by combining a xc2xd wavelength phase difference film with a xc2xc wavelength phase difference film, both being made from a norbornene polymer, formed between the polarizer and the transparent insulating substrate; and
wherein a twisted direction of the liquid crystal occurring when the liquid crystal is traced from a side of the facing substrate to a side of the liquid crystal driving element forming substrate relative to an oriented direction, which serves as a reference of an angle to be formed, on a side of the facing substrate on the liquid crystal is defined as being xe2x80x9cpositivexe2x80x9d and an angle xe2x80x9cxcex1xe2x80x9d formed by a light absorbing axis of the polarizer and by the oriented direction is set to be within a range of 31 degrees to 41 degrees, an angle xe2x80x9cxcex2xe2x80x9d formed by an optical axis of the xc2xd wavelength phase difference film and by the oriented direction is set to be within a range of 17 degrees to 27 degrees, and an angle xe2x80x9cxcex3xe2x80x9d formed by an optical axis of the xc2xc wavelength phase difference film and by the oriented direction is set to be within a range of xe2x88x9234 degrees to xe2x88x9224 degrees and permitivity anisotropy of the liquid crystal is set to be about 6 degrees or more.
In the foregoing first aspect, a preferable mode is one wherein the permitivity anisotropy of the liquid crystal is set to be within a range of 6 to 14.
Also, a preferable mode is one wherein the twisted angle of the liquid crystal is set to be from 66 degrees to 74 degrees and a product of anisotropy of refractive index of the liquid crystal and a thickness of a layer of the liquid crystal is set to be within a range of 0.21 xcexcm to 0.31 xcexcm.
Also, a preferable mode is one wherein the xc2xd wavelength phase difference film and the xc2xc wavelength phase difference film are made from a material exhibiting small wavelength dispersion in anisotropy of refractive index in a visible light region.
Also, a preferable mode is one wherein the reflective electrode is formed so as to have bumps and dips on its surface.
In addition, a preferable mode is one wherein the norbornene polymer includes Arton(trademark).
According to a second aspect of the present invention, there is provided a method for manufacturing a reflective-type LCD for obtaining a desired display by forming a liquid crystal between a liquid crystal driving element forming substrate and a facing substrate and by causing light incident on the liquid crystal from a side of the facing substrate to be reflected on a side of the liquid crystal driving element forming substrate and to be emitted from a side of the facing substrate so as to be observed, the method including:
a process of forming the liquid crystal driving element forming substrate including a liquid crystal driving element, a reflective electrode, and a first oriented film formed respectively on an insulating substrate;
a process of forming the facing substrate in which a common electrode made up of a transparent conductor and a second oriented film on a side opposite to the insulating substrate on a transparent insulating substrate are provided;
a process of injecting the liquid crystal between the liquid crystal driving element forming substrate and the facing substrate so that the liquid crystal comes into contact with the first and second oriented films and causing the liquid crystal to be oriented, in accordance with a rubbing angle formed in advance on the first and second oriented films, so that a twisted angle is set to be within a range of 66 degrees to 74 degrees and that a product of anisotropy of refractive index of the liquid crystal and a thickness of a layer of the liquid crystal is set to be within a range of 0.21 xcexcm to 0.31 xcexcm; and
a process of forming a polarizer on a side opposite to a side being in contact with the liquid crystal on the facing substrate with a layer-stacked type xc2xc wavelength plate constructed by combining a xc2xd wavelength phase difference film and a xc2xc wavelength phase difference film, both including a norbornene polymer.
In the foregoing second aspect, a preferable mode wherein, in the liquid crystal injecting process, the liquid crystal is injected in a manner that a twisted direction of the liquid crystal occurring when the liquid crystal is traced from a side of the facing substrate to a side of the liquid crystal driving element forming substrate relative to an oriented direction, which serves as a reference of an angle to be formed, on a side of the facing substrate on the liquid crystal is defined as being xe2x80x9cpositivexe2x80x9d and an angle xe2x80x9cxcex1xe2x80x9d formed by a light absorbing axis of the polarizer and by the oriented direction is set to be within a range of 31 degrees to 41 degrees, an angle xe2x80x9cxcex2xe2x80x9d formed by an optical axis of the xc2xd wavelength phase difference film and by the oriented direction is set to be within a range of 17 degrees to 27 degrees, and an angle xe2x80x9cxcex3xe2x80x9d formed by an optical axis of the xc2xc wavelength phase difference film and by the oriented direction is set to be within a range of xe2x88x9234 degrees to xe2x88x9224 degrees and permitivity anisotropy of the liquid crystal is set to be about 6 degrees or more.
Another preferable mode is one wherein the liquid crystal having permitivity anisotropy of being within a range of 6 to 14 is used.
Still another preferable mode is one wherein in the polarizer forming process, as said norbornene polymer, Arton(trademark) is used.
With the above configuration, since the liquid crystal is sandwiched between the TFT substrate and the facing substrate, since the polarizer is formed on the side opposite to the side being in contact with the liquid crystal on the second transparent insulating substrate with the layer-stacked xc2xc wavelength plate constructed by combining the xc2xd wavelength phase difference film with the xc2xc wavelength phase difference film both being made from a norbornene polymer, being sandwiched between the polarizer and the second transparent insulating substrate, and since the permitivity anisotropy of the liquid crystal is set to be about 6 degrees or more, the wavelength dispersion in anisotropy of the refractive index in a visible light region can be made smaller and the contrast can be made higher without causing an increase of the driving voltage.
With another configuration, by a combination of well-known processes, without the use of additional special processes, the reflective-type LCD can be manufactured and, therefore, no rise in costs occurs. Moreover, development of unwanted colors when a black color is displayed can be avoided and high contrast can be stably obtained in the display.